1. Technical Field
The present disclosure relates to failure detecting apparatuses that detect a failure of a signal detection apparatus, and more particularly to a failure detecting apparatus that detects an isolation failure of the signal detection apparatus that detects voltage of a plurality of coils magnetically coupled therebetween.
2. Description of the Related Art
The signal detection apparatus at which the failure detecting apparatus diagnoses whether or not an isolation failure exists, includes one or more primary coils to which alternating current (AC) signal is applied and one or more secondary coils magnetically coupled to the primary coils. The signal detection apparatus detects voltage at the secondary coils.
Particularly, as an example of a failure detecting apparatus, Japanese Patent No. 4122606 discloses a failure detecting apparatus adapted to a resolver as a signal detection apparatus as follows. In the resolver, a magnetic flux is generated by an AC excitation current applied to the primary coil that rotates with the rotor whereby voltage is induced at the secondary coil and therefore, a rotating speed of the rotor is detected by the resolver. The resolver includes the primary coil and a pair of secondary coils which are isolated each other, however, if these coils are short-circuited, a detection accuracy of the rotation speed is decreased. The failure detecting apparatus according to the above patent document is designed to diagnose whether or not a short-circuit has occurred such that individual bias voltage are applied to the primary coil and the secondary coils and whether or not a short-circuit exists is diagnosed based on whether the direct current (DC) potential of the coil with respect to the ground is the bias voltage or more.
However, in the above-described failure detecting apparatus, setting a threshold value in order to determine whether or not the DC potential with respect to the ground is the bias voltage or more depends on the bias voltage. Hence, to flexibly set the threshold value, it is necessary to change the bias voltage. In contrast, when the bias voltage is set in other respects, a range of the threshold value that can be set is narrowed so that a disadvantage such as degrading noise tolerance is likely to occur.